Đề tài mô phỏng hệ thống thu phát tín hiệu GPS trên Matlab. Đề tài được thực hiện bằng công cụ mô phỏng Simulink. Đồ án được viết bằng tiếng anh hết sức chi tiết và dễ đọc. Mong sẽ giúp các bạn sinh viên, học viên cao học đang quan tâm về vấn đề này.
Trang 1Global Positioning System
Signal Simulation
A Thesis presented for the Degree of Bachelor of Electrical
Engineering (Honours)
By Thiam Hock Tan
Department of Information Technology and Electrical Engineering
The University of Queensland
Australia October 29, 2003
Trang 2Dear Professor Simon Kaplan,
In accordance with the requirements of the degree of Bachelor of Engineering (Honours)
in the division of Electrical Engineering, I present the following thesis entitled
“Global Positioning System Signal Simulation”
This thesis was performed under the supervision of Professor Kurt Kubik and supervised by Dr Adam Postula
co-I certify that this thesis is my own and co-I acknowledged that it does not contain any material previously published for a degree at the University of Queensland or any other institution, except where acknowledged and referenced
Yours sincerely,
Thiam Hock Tan
Trang 3Acknowledgements
Acknowledgements
There are a number of people that I would like to give special thanks, who have contributed to the successful completion of this thesis My apologies are extended to anyone whom I may have inadvertently missed
I would like to express my sincere appreciation to Professor Kurt Kubik and Dr Adam Postula for assisting and guiding me along this thesis Without their help, this thesis will not reach the current stage and I greatly thank them for their precious time and efforts committed in the accomplishment of this thesis
I would like to thank Mr Bradley Houston for assisting me and helping me out regarding my doubts in MATLAB (Simulink) and help me clarify them
I also wish to thank my team mate, Mr Wai Cheng Yong who dedicated his time
in the hardware so that I can concentrate in the software of this thesis to the completion
of this thesis
Special thank is express to Mr Chin Chye Neo who stay up late with me in completing my thesis as we assist one another in our thesis, he is working on the “Ultra Wideband (Positioning)” thesis He is a great motivator and has a strong sense of humour
in times of difficulties to make us forget the present problems and work towards our final goal
Not forgetting to thank my dearest parents, Mdm Chee Kim Foo and Mr Eng Foo Tan who offer me the best education and taking so much pain and effort in guiding me till today No words can represent my deepest gratitude for them I just want to say, “It’s all within my heart”
Trang 4Abstract
Abstract
Due to the modernization and advance technological advancement, days of using the street directory for navigation are soon going to be obsolete Global Positioning System is the “Gateway to the Future” This GPS Signal Simulator has the ability to simulate the current GPS L1 frequency at the user’s fingertips
Realistic generation of this L1 frequency allows the user to have a deeper understanding of what exactly the GPS signal is about and how it is coded and modulated Coding of this signal is done by using the Coarse Acquisition (C/A) code Data will spread across this C/A code before mapping onto the Binary Phase Shift Keying (BPSK) signal This BPSK signal is modulated onto a carrier frequency to be transported along the communication channel
To be able to receive without any signal variations is almost impossible due to the extensive distance between the satellite and the receiver unit Therefore, demonstration of signal variation can be done by using the Doppler shift in this simulation
Doppler Shift is the variation of frequency along the communication channel and
it greatly depends on the location of the satellite If the satellite is just above the user and
at the closest position relative to the user, the Doppler frequency is zero But other than this position, all other position involves Doppler shift and thus signal variation occurs
Other than Doppler shift, this simulation can observe the autocorrelation of any of the 32 satellite’s available in the GPS constellation right now Delay of any amount of chips can be simulated to show the autocorrelation of the lag function
With this simulation, GPS L1 signal can be analyzed and observed to assist in any future GPS development
Trang 5Contents
Contents
Candidate’s Declaration ……… ……… i
Acknowledgements ……… ii
Abstract ……….…… iii
List of Figures ……… …… ix
List of Tables ……… xi
Chapter 1 Introduction ……… 1
1.1 Overview of thesis ……….… 2
1.2 Implementation ……… 2
1.3 Desired results ……… 3
1.4 Organization of Thesis ……… 4
Chapter 2 Literature Review ……… 7
2.1 Global Positioning System ……… 7
2.1.1 Control Segment ……… 8
2.1.2 Space Segment ……… 8
2.1.3 User Segment ……… 9
2.2 GPS Signal Structure ……… 9
2.3 Determine User’s Location ……… 10
2.4 Pseudorange Measurement ……… … 11
2.5 Coarse Acquisition Code (C/A code) ……… 12
2.6 Autocorrelation ……… 14
2.7 Doppler Shift ……… 16
2.8 Binary Phase Shift Keying (BPSK) ……… 20
2.9 Data Message Format ……… 20
2.10 Summary of Chapter 2 ……….… 21
Trang 6Contents
Chapter 3 GPS Signal Simulator in the Market ……… 23
3.1 Accord GPS Correlator Simulator ……… … 23
3.2 Welnavigate GS 600 ……… 25
3.3 CAST 1000 Satellite Signal Simulator ……… 27
3.4 Spirent Multi-Channel GPS/SBAS Simulation System STR4500 … … 28
3.5 Summary of Chapter 3 ……….… 29
Chapter 4 Simulation of GPS L1 Signal Generator ……… 31
4.1 Block Description ……… 32
4.2 S-function ……… 33
4.3 MATLAB function ……… 34
4.3.1 MATLAB function (Right) ……… 35
4.3.2 MATLAB function (Counter1023mega2) ……… 35
4.3.3 MATLAB function (Opmega) ……… 35
4.4 Simulation of Part One ……….… 36
4.4.1 Block 1 (Assign empty matrix) ……… 37
4.4.2 Block 2 (50 bits data) ……… 37
4.4.3 Block 3 (Data input) ……… …… 37
4.4.4 Block 4 (Sfunsv20460) ……… … 37
4.4.5 Block 5 (Spreading block) ……… 38
4.4.6 Block 6 (SV 1 to 32) ……… 38
4.4.7 Block 7 (SV Selector) ……… 38
4.4.8 Block 8 (To file) ……….… 38
4.5 Interior of Part One Block 5 (Spreading Block) ……… … 39
4.5.1 Block 1 (NA) ……… 39
4.5.2 Block 2 (For Iterator) ……… 39
4.5.3 Block 3 (Goto i) ……… 40
4.5.4 Block 4 (Port 1) ……… … 40
4.5.5 Block 5 (Port 2) ……… 40
4.5.6 Block 6 (Selector) ……… 40
4.5.7 Block 7 (tmp_col) ……… 40
Trang 7Contents
4.5.8 Block 8 (Port 3) ……… 40
4.5.9 Block 9 (num_B_eles) ……… 41
4.5.10 Block 10 (ones(size(u))) ……… 41
4.5.11 Block 11 (Mux) ……… 41
4.5.12 Block 12 (double(u(1):u(2))) ……… 41
4.5.13 Block 13 (Port 1) ……… 41
4.6 Interior of Part One Block 6 (SV 1 to 32) ……… 42
4.7 Simulation of Part Two ……… 43
4.7.1 Block 1 (Input 1M C/A XOR Data) ……… 44
4.7.2 Block 2 (Fc = 10.23 MHz) ……….…… 44
4.7.3 Block 3 (BPSK Modulation) ……… 44
4.7.4 Block 4 (10 KHz Doppler Shift) ……… 44
4.7.5 Block 5 (Selector) ……… 44
4.8 Interior of Part Two Block 3 (BPSK Modulation) ……… … 45
4.8.1 Block 1 (In 1) ……… 45
4.8.2 Block 2 (Constant) ……… 45
4.8.3 Block 3 (Switch) ……… 45
4.8.4 Block 4 (Step) ……… 46
4.8.5 Block 5 (Sum) ……… 46
4.8.6 Block 6 (Inverse) ……… 46
4.8.7 Block 7 (In 2) ……….… 46
4.8.8 Block 8 (Modulation) ……….… 46
4.8.9 Block 9 (Out 1) ……… 46
4.9 Interior of Part Two Block 4 (10 KHz Doppler Shift) ……… 47
4.9.1 Block 1 (In 1) ……….… 47
4.9.2 Block 2 (Product) ……… 47
4.9.3 Block 3 (Relay) ……… 47
4.9.4 Block 4 (Multi-port Switch) ……… 48
4.9.5 Block 5 (In 2) ……… 48
4.9.6 Block 6 (Product1) ……… 48
4.9.7 Block 7 (Out 1) ……… … 48
Trang 8Contents
4.10 Part One of Autocorrelation Simulation ……… 48
4.10.1 Block 1 (SV Selector) ……… 49
4.10.2 Block 2 (SV 1 to 32) ……… 49
4.10.3 Block 3 (Autocorrelation) ……… 49
4.10.4 Block 4 (Timing) ……… 49
4.10.5 Block 5 (Reshape) ……… 49
4.10.6 Block 6 (To Workspace) ……… 50
4.11 Interior of Part One Autocorrelation Block 2 (SV1 to 32) ……… 50
4.12 Part Two of Autocorrelation Simulation ……… 51
4.12.1 Block 1 (From Workspace) ……… 51
4.13 Summary of Chapter 4 ……… 51
Chapter 5 User Manual ……… 53
5.1 System Requirements ……… 53
5.2 Getting the Files and Folders in the Right Location ……… 54
5.3 Simulation Procedures for Part_1 ……… 55
5.4 Simulation Procedures for Part_2 ……… 56
5.5 Simulation Procedures for Autocorrelation_1 ……… 58
5.6 Simulation Procedures for Autocorrelation_2 ……….… 58
5.7 Summary of Chapter 5 ……… 59
Chapter 6 Simulation Results and Analysis ……… 61
6.1 Data Stream of L1 Signal ……… 61
6.2 Carrier Frequency of L1 Signal ……… 62
6.3 Converted Data Stream of L1 Signal ……… 63
6.4 BPSK Data Stream of L1 Signal ……… … 64
6.5 L1 Signal ……… 65
6.6 Doppler Shift of L1 Signal ……… 66
6.7 Frequency Spectrum of Carrier ……… 67
6.8 Frequency Spectrum of L1 Signal ……… 68
6.9 Autocorrelation Function of Space Vehicle ……….… 70
Trang 9Contents
6.10 Problems Encountered and Rectification ……… 71
6.11 Summary of Chapter 6 ……….… 74
Chapter 7 Conclusion ……… … 75
7.1 Lessons learnt ……… 76
7.2 Further Development of this Simulator ……… 76
Appendices ……… 77
Appendix A Sfun50 ……… 77
Appendix B Sfunsv20460 ……… 78
Appendix C Sfunsv1000 ……… 80
Appendix D Right ……….… 81
Appendix E Counter1023mega2 ……… 82
Appendix F Opmega ……… 89
Appendix G Sfunsv1 ……… 90
Appendix H Sfunxcorr ……… 92
Appendix I Sfuntime ……… … 94
References ……… 96
Trang 10List of Figures
List of Figures
Figure 1.1: Picture of a satellite in space ……… … 2
Figure 2.1: GPS constellation ……… 7
Figure 2.2: GPS satellite signal ……… 10
Figure 2.3: 4 satellites to get exact user’s location ……….… 11
Figure 2.4: Pseudorange measurement ……… … 11
Figure 2.5: GPS C/A code generator ……… 14
Figure 2.6: An Autocorrelation Function of SV 10 ………… ……… 16
Figure 2.7: Doppler Principle ……… 17
Figure 2.8: Doppler Calculation ……… 18
Figure 4.1: S-function block ……… … 33
Figure 4.2: MATLAB function block ……… 34
Figure 4.3: Block diagram of Part One ……… 36
Figure 4.4: Part One of Simulation ……… 36
Figure 4.5: Spreading Block in Part One ……… 39
Figure 4.6: SV 1 to 32 Block in Part One ……… 42
Figure 4.7: Block diagram of Part Two ……… 43
Figure 4.8: Part Two of Simulation ……… 43
Figure 4.9: BPSK Modulation Block in Part Two ……… 45
Figure 4.10: 10 KHz Doppler Shift Block in Part Two ……… 47
Figure 4.11: Simulation of Autocorrelation Part One ……… … 48
Figure 4.12: SV 1 to 32 Block from Autocorrelation Part One ……… 50
Figure 4.13: Simulation of Autocorrelation Part Two ……… 51
Figure 6.1: Data Stream from Part One of Simulation ……… 61
Figure 6.2: Carrier Frequency of 10.23 MHz ……… 62
Figure 6.3: Converted Data of Data Stream ……… … 63
Figure 6.4: BPSK Data format of Data Stream ……… 64
Figure 6.5: L1 signal of SV1 ……… 65
Figure 6.6: Doppler Shift of L1 Signal ……… … 66
Trang 11List of Figures
List of Figures
Figure 6.7: Frequency Spectrum of Carrier ……… … 67
Figure 6.8: Frequency Spectrum of L1 Signal ……… … 68
Figure 6.9: Overall View of the Frequency Spectrum of L1 Signal ………… 69
Figure 6.10: Base Band Frequency Spectrum of C/A code ……… 69
Figure 6.11: Autocorrelation Function of SV 10 with a “Lag” of 25 chips ………… 70
Trang 12List of Tables
List of Tables
Table 2.1: GPS C/A Code Assignments ……… … 13
Table 2.2: C/A Code Autocorrelation Parameters ……… 15
Table 2.3: BPSK Mapping Scheme ……… 20
Table 4.1: S-function Flag Format ……… 34
Trang 13To non - military users, GPS appears to be a navigation tool for hiking and finding your current location Whereas for military users, GPS is very important as it plays a vital part in whether you win or lose in a war, as precise and definite location must be known Any slight error may cause dramatic effects Therefore, GPS usage is very wide and depends on the individual user to use it appropriately
Due to the technological advances and advancement of the human race, the need for accurate and immediate decision stimulated the boom of the GPS industries GPS receivers used to cost a “bomb” in the past, but due to great demand causing great supply, therefore current cheapest GPS receiver cost only ten dollars
Public transport nowadays uses GPS for navigation and receiving message from their control centre The GPS not only integrated into our lifestyle, but also has even become indispensable Faster pace of life requires quick responses and immediate reaction Consumers are the reason why the GPS is getting popular in our society
Trang 14to be able to finish the assigned task efficiently and effectively
At the end of this thesis, both software and hardware will be able to complement one another and deeper understanding between the two areas provide a very good opportunity
to learn in the above two aspects Precious experiences are gathered throughout this thesis and valuable knowledge acquired
1.2 Implementation
Software implementation will be done on MATLAB (Simulink) platform All simulations
in this thesis are real values from the satellites, unless otherwise stated The binary data
Trang 15Chapter 1 Introduction
are coded with coarse acquisition code, commonly known as C/A code before mapping onto a binary phase shift keying (BPSK) signal with a carrier frequency, simulating the transmission of data over the channel Certain affecting attributes, for instance Doppler shift will be applied to the BPSK waveform to see the resultant effect as in real life system Although the receiver is not required in this thesis, but autocorrelation is done so that some aspects of the receiving end will be learned through this process
Hardware implementation is to design and fabricate the frequency synthesizer for the GPS signal simulator The main concern is to generate the L1 signal carrier frequency
at 1575.42 MHz carrier frequency and to be able to lock this frequency using the phase lock loop Details on the hardware development and the fabrication process of the frequency synthesizer can be found in the thesis “GPS Signal Simulator” written by my partner, Yong Wai-Cheng [3]
1.3 Desired results
The intended result is to build a GPS signal simulator But due to the limitation of equipments and human resource, the thesis scope change slightly to suit our present settings For software development, creation of the GPS L1 signal generator is required using MATLAB Simulink platform Whereas for the hardware development, designing and building the frequency synthesizer is the main task involved Tasks are broken down into different modules, so that different modules can be connected together to form the whole signal simulator
The GPS signal simulator is currently available in the market But due to the specialization in this particular area of interest, each GPS signal simulator could cost up
to tens of thousands or even hundred of thousands U.S dollars Therefore the knowledge acquired in this thesis is very valuable in today’s market due to the intense specialization
Trang 16Chapter 1 Introduction
1.4 Organization of Thesis
In this thesis we report on the simulation of Global Positioning System (GPS) signal L1 using MATLAB software This software is particularly useful in technical computing and simulating program With the help of Simulink, which is a model based and system level design in real time simulation of the GPS signal can be carried out realistically
The topics layout for each chapter is as follows In chapter 1, the GPS signal will
be introduced and briefly discussed to let the reader have a basic understanding before going into further details, the implementation, followed by the purpose of this thesis The initial plan that this thesis is going to achieve and the actual results achieved Chapter 2 will consist of a comprehensive review on GPS The background knowledge and relevant studies will be presented for a clearer picture Topics covering in this thesis will be enhanced with detailed descriptions on individual topic to have a better understanding
Chapter 3 analyses the GPS signal simulator currently available in the market and their individual functions and cost The main objective is to tap into the vast area of lucrative market of GPS In chapter 4, the simulation of the GPS signal generator will be discussed in details Due to the limitations of the software and the computer that the software is running on, certain constraints are present and made known Moving in depth into MATLAB (Simulink) block sets and how they are incorporated in this thesis simulation M files and S-functions are the two main components in this section that requires much attention Individual block will be discussed and described in details
Chapter 5 is the user manual for the Simulink simulation This chapter provides comprehensive steps for the user to operate the simulation The steps for individual part
of the programs are described fully and the user only needs to follow the instructions and operate accordingly Blocks that allow changes are specified in the manual to assist the user so that the programs would be able to maximize and enhanced the simulation to the fullest potential
Trang 17Chapter 1 Introduction
In chapter 6, the MATLAB (Simulink) results will be analyzed and compared with the theoretical findings The problems encountered during the simulation process will be addressed and rectified accordingly
Lastly, chapter 7 is the conclusion for this thesis Whereby all the lessons and knowledge gathered from this thesis will be acknowledge and any future plans to carry on the thesis
Trang 19Chapter 2 Literature Review
Chapter 2
Literature Review
This chapter will discuss about the relevant background knowledge and information required in the undertaking of this thesis
2.1 Global Positioning System
As of updated on September 16, 2003 there are 31 satellites presently in space orbiting But only 24 satellites are in used, the rest are spares [4] They are arranged in 6 orbital planes with 4 satellites each plane There are 3 segments in GPS, namely control, space and user segment
Figure 2.1 – GPS constellation [4]
Trang 202.1 Global Positioning System
2.1.1 Control Segment
Master Control Station – This station is located at Falcon Air Force Base in Colorado
Springs, Colorado It controls the overall management of the remote monitoring and transmission sites As the main centre for support operations, it calculates any position or clock errors for each individual satellite according on the information received from the monitor stations If error is found, it will “order” the appropriate ground antenna to relay
the requisite corrective information back to that particular satellite [5]
Monitor Stations – There are altogether five monitor stations located at Falcon Air
Force Base in Colorado, Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in the South Pacific Ocean The monitor stations checks the exact altitude, position, speed, and the overall health of the orbiting satellites The control segment uses the information collected by the monitor stations to predict the behaviour of individual satellite’s orbiting and clocking status, making sure that they remain in acceptable limits The prediction data is up linked to the satellites for transmission back to the users A station can perform a track up of up to 11 satellites at a time This “check up” is performed twice a day as the satellites complete their journeys
around the earth [5]
Ground Antenna – Ground antennas are used to monitor and track the satellites They
also transmit corrective information to back to the individual satellites [5]
2.1.2 Space Segment
The space segment consists of the satellites and the Delta rockets that launch the satellites from Cape Canaveral, in Florida The satellites move in circular orbit at an altitude of 10,900 miles (17,500 km) with a period of 12 hours The orbits are tilted to the earth’s equator by 55 degrees to ensure the coverage of the polar regions As they are powered
by the solar cells, the satellites will orientate themselves to face toward the sun for power
Trang 212.1 Global Positioning System
and their antennas toward the earth for transmission There are a total of 24 satellites positioned in 6 orbital planes to ensure coverage of the entire earth [6]
2.1.3 User Segment
The user segment implies to all the users of the global positioning system They can be classified into two groups, military users and non – military users The military uses the GPS in a wide range of scope, from navigation tools to target designation, air support to the integration of smart weapons [7] For the civilians, GPS are used in daily life applications, providing point-to-point navigation in public bus services, navigational usage by the hikers and mountaineers, and many more With the integration of the GPS in our life, the GPS will be a cornerstone of the future air traffic management (ATM) Providing a high degree of safety and precision while reducing delays and increasing airway capacity [7]
2.2 GPS Signal Structure
The satellites broadcast ranging codes and navigation data on two frequencies using a technique called Code Division Multiple Access (CDMA) [8] There are only two frequencies in use by the system, L1 (1575.42 MHz) and L2 (1227.6 MHz) In this thesis, only L1 is used, as we concentrate only on the primary frequency and not the secondary frequency L2 Equation of L1 is as follows,
SL1 (t) = G1 C/A SV(t) d(t) cos (Wc t) + G2 PSV(t) d(t) sin (Wc t) [8] The fundamental frequency (fo) is 10.23 MHz, L1 is 154 times fo and L2 is 120 times fo
Trang 222.2 GPS Signal Structure
Figure 2.2 – GPS satellite signal [2]
2.3 Determine User’s Location
The GPS uses 3 satellites to capture the user’s location and the fourth satellite to get the precise timing so that the distance of the user can be tracked and derived The basic navigation method measures the signal delay between 3 satellites and the user equipment, computes from these the ranges to form 3 spheres, and their intersection position indicate the user’s position A fourth satellite is required to perform the calculation, as time is the fourth unknown factor therefore requiring a fourth input for explicit solution The need for the fourth satellite is due to the almost perfect atomic clock that is being used in the satellite-timing signal compared to the normal clock in the receiver The atomic clock is much more precise then the clock used in the GPS receiver Any slight error in the user clock could results in a huge difference in the exact location It is extremely uneconomical for every GPS receiver to contain an atomic clock which cost around 50 to
100 thousands U.S dollars, therefore the fourth satellite come into place
Trang 23
2.3 Determine User’s Location
Figure 2.3 – 4 satellites to get exact user’s location [2]
2.4 Pseudorange Measurement
The location of the user is obtained by using pseudorange measurement The measurement of the receiver is achieved by recording the actual time taken for the relevant code to travel from the satellite to the user’s receiver Multiply this actual time recorded by the speed of light to convert this timing into actual distance The process for this measurement is in operation when the receiver picks up the signal from the satellite and compares the incoming signal to the internally generated C/A code, the difference in time is the travel time for the signal from the satellite to the receiver
Travel Time x Speed of Light
Figure 2.4 – Pseudorange measurement [2]
Trang 242.5 Coarse Acquisition Code (C/A code)
2.5 Coarse Acquisition Code (C/A code)
There are 37 pseudo random noise (PRN) sequences used for the C/A codes Each satellite (SV) has its individual C/A code The C/A code is also known as the Gold code, the code has good auto and cross correlation properties The cross correlation of the Gold code is such that the correlation function between two different sequences is low Every satellite broadcasts a different code, repeating it over and over again It contains no actual data; it is simply an identifier These codes are binary, consisting of “zeros” and “ones” Each individual zero and one is called a chip instead of a bit because they contain no data They are of a fixed pattern and length that are repeated indefinitely The C/A code is
1023 chips long with a broadcasting frequency of 1.023 Mega chips per second It is repeated every millisecond, and each chip is 293m (or 0.978 microsecond) long The whole sequence is about 300km long [9]
Two shift registers are needed to generate the C/A code They are G1 and G2, which represents shift register one, and shift register two respectively The polynomial for register G1 is 1 + X3 + X10 and the polynomial for register G2 is 1 + X2 + X3 + X6 +
X8 + X9 + X10 Using this two shift registers, the C/A code can be generated as shown in Figure 2.5 The initial conditions for these two registers are set to all ones
But for the case of C/A code, not all the bits in register G2 are used Specific bits from register G2 are tapped for different satellite Satellite is also known as Space Vehicle (SV) The bits that are to be tapped for different space vehicles are shown in Table 2.1
Trang 252.5 Coarse Acquisition Code (C/A code)
Table 2.1 – GPS C/A Code Assignments [10]
The first 10 chips in Table 2.1 are used to verify the generated C/A code, to
ensure that they are correctly generated They are represented in octal notation Take SV
1 for example The binary representation is 1100100000, which is the octal representation
equivalent of 1440, that is specified in most GPS textbook If the first 10 chips of the
generated code for SV1 tallies with the first 10 bits of SV1 as describe above, then the
generated code is proved to be correct
Trang 262.5 Coarse Acquisition Code (C/A code)
Figure 2.5 – GPS C/A code generator [2]
2.6 Autocorrelation
The autocorrelation characteristics of the GPS gold codes are fundamental to the signal demodulation process The correct distance of the user is dependable on this process The time difference is multiplied by the speed of light to get the user’s location as described
in the earlier section A replica of the C/A code is produced at the receiver’s side and constantly shifting in phase to match the received C/A code The point to which both the C/A code match perfectly is the time delay for the signal to travel from the satellite to the receiver Autocorrelation is mainly to detect the time delay (τ) of the signal The autocorrelation function of the GPS C/A code is as follows,
[8]
Trang 272.6 Autocorrelation
Gi (t) = C/A code Gold code sequence as a function of time t for SVi
TCA = C/A code chipping period (977.5 nsec)
τ = Phase of the time shift in the autocorrelation function
If the signals are unmatched, the sum of products of the code state is close to zero, but if
the signals are matched, the sum of products of the code state is exactly 1
Typical autocorrelation amplitude outside
1
Typical autocorrelation (outside the
correlation interval) in dB with respect to
maximum correlation
- 30.1
Autocorrelation time interval (nsec) 1955.0
C
R = Chipping rate (chips/sec) 1.023×106
C
Table 2.2 – C/A Code Autocorrelation Parameters [8]
Trang 292.7 Doppler Shift
Figure 2.7 – Doppler Principle [12]
Trang 302.7 Doppler Shift
The effect of Doppler shift in GPS is shown in Figure 2.7 It indicates that when the satellite is just above the user, the Doppler shift is zero But if the satellite is at point
S1 or S2, positive Doppler Shift occurs On the other hand, if the satellite is at point S4, S5
or S6, negative Doppler shift will occur The calculation of Doppler shift other than the point at which the satellite is just above the user is shown below [13]
Figure 2.8 – Doppler Calculation [13]
Trang 312.7 Doppler Shift
Applying (2),
)(
21
t
−+
∆
=
By the Mean Value Theorem, the differential approximation introduces a relative error of
no more than 2V2∆t/Rc=2V2λ/Rc2 =8×10− 17 which is entirely negligible
To get from (3) to the standard formula,
The approximation in (5) has a relative error of
(where we use ), again negligible Finally, we attain the Doppler shift,
13 2
2
2 (2 / ) 2 10)
/2( R& c = V t Rc ≤ × −
R t V
R& = 2 /
R R
c
f f
f
f D
λ
22
'− =− =−
Trang 322.8 Binary Phase Shift Keying (BPSK)
2.8 Binary Phase Shift Keying (BPSK)
Binary Phase Shift Keying is modulating the digital bits onto a carrier frequency using only two phases, an In - Phase and the other 180° Out - Of - Phase The binary bits are mapped as shown in the table below
Table 2.3 – BPSK Mapping Scheme
The reason for the implementation of BPSK in GPS is due to the nature that BPSK is less susceptible to noise and therefore it helps in maintaining the correct information being passed through the communication channel
2.9 Data Message Format
The GPS navigation message content consists of constellation and specific satellite information Constellation information made up of almanac data, health data and ionospheric corrections model for all satellites Whereas the specific satellite information consists of ephemeris, clock corrections and synchronization data [14]
A message structure consist of 5 sub frames, each sub frame has 10 to 30 bits words at 50 Hz (300 Bits) Each of the 30 sub frame words also contains 6 parity bits The first word is TLM with 8-bit preamble for data synchronization and the second word
is HOW Data block (I) consist of sub frame I Data block (II) consist of sub frames 2 and
3 Data block (III) consist of sub frames 4 and 5 (almanac) [14]
The basic message unit for one frame is 1500 bits long One frame consists of five sub frames One sub frame consists of 10 words and one word consists of 30 bits One
Trang 332.9 Data Message Format
Master frame includes all 25 pages of sub frames 4 and 5 (37 500 bits taking up to 12.5 minutes) Sub frames 1, 2 and 3 repeat as pages of sub frames 4 and 5 increments Sub frame 1 includes the flags (L2 code and data; week number; satellite accuracy and heath), the age of data and the satellite clock correction coefficients Sub frame 2 and 3 are the orbit parameters Sub frame 4 contains more information, such as almanac for satellites
25 – 32 (pages 2,3,4,5,7,8,9,10), ionospheric model, and UTC data (page 18), antispoof flag 25 – 32 satellites (page 25), satellite configuration 25 – 32 satellites (page 25), health
of satellites 25 – 32 (page 25), reserved (pages 1,6,11,12,16,19,20,21,22,23,24), spare (pages 13,14,15) and special message (page 17) Sub frame 5 contains almanac for satellites 1 – 24 (pages 1 – 24), and health of satellites 1 – 24 (page 25) [14]
2.10 Summary of Chapter 2
This chapter discussed about the necessary background information about the basic GPS functions, and all that is needed to understand the rest of the chapters The knowledge gathered from this chapter creates the awareness for the reader about the chapters ahead and what to expect from the following chapters The next chapter will discuss about the GPS signal simulators that are currently in the market and what are the usual specifications and some of the prices of the simulators The discussion will cover both hardware and software simulations
Trang 35Chapter 3 GPS Signal Simulator in the Market
Chapter 3
GPS Signal Simulator in the Market
GPS Signal Simulator is currently available in the commercial market But due to the specialization in this particular field, this type of simulator cost a huge amount of money and is used only by the company developing GPS receiver for simulating the signal characteristics of GPS to test if their receiver is working under certain environment, or by organizations studying the signal characteristics of GPS In the following, a selected number of simulators are listed
3.1 Accord GPS Correlator Simulator
This Simulator is a very powerful software package for understanding the characteristics
of the GPS satellite signal and it’s processing in a typical GPS receiver It provides a sound platform to experiment with the various modules that constitute the signal processing section of a GPS receiver [15]
The signal simulator consists mainly on two major modules, which are the input signal simulator and the signal processor The input signal simulator generates the GPS signal spectrum according to the user’s specifications at a considerably low intermediate frequency Whereas the signal processor converts the incoming modulated intermediate frequency to in phase and quadrature signals in a costas loop The code and carrier phase lock loops perform signal tracking [15]
The GPS signal acquisition and tracking process can be analyzed using the input signal simulator and the signal processor The different components of the Accord’s correlator allow the user to study and have a deeper understanding on their effect on performance, either individually or collectively [15]
Trang 363.1 Accord GPS Correlator Simulator The functions of this correlator are as follows,
Generation of C/A code
Separation of In phase and Quadrature signals in a costas loop
Computation of auto and cross correlation values
Signal acquisition using cell by cell search
Signal tracking
Computation of Doppler and Code phase
Input signal Simulator
Generation of C/A codes for all GPS satellites
Selection of a low IF (1.0 MHz to 6.0 MHz)
Introduction of Doppler Shift (-10 KHz to 10 KHz)
Variation of carrier phase (0° to 180°)
Introduction of C/A chip shift
Addition of Gaussian noise (-90 dB < SNR < +90 dB)
Signal Processor
Generation of C/A codes for all GPS satellites
Selection of sampling frequency (1 to 7 MHz)
Selection of code type (Ideal or DCO clocked)
Variation of DCO resolution (code and carrier)
Generation of local carrier (1.0 to 6.0 MHz)
Introduction of estimated Doppler Shift (-10 KHz to +10 KHz)
Variation of local carrier phase (0° to 180°)
Introduction of C/A code chip and phase shift
Selection of C/A code chip and phase shift
Selection of Integration period (1 to 20)
Currently this Accord GPS correlator simulator is selling at $495 US dollars [15]
Trang 373.2 Welnavigate GS 600
3.2 Welnavigate GS 600
The GS600 is a 6 to 8 channels L1 – C/A code simulator capable of generating a RF signal corresponding to the GPS satellites On this machine, the necessary data to run the simulation is stored on a PC card Therefore, computer is not needed to run alongside with this hardware as it can function on itself This unit comes with two PC card scenarios Firstly is the three-hour static position and secondly is the three-hour racetrack
If a new scenario is needed, it can be ordered from Welnavigate The GS600 is currently selling for $24,900 Delivery time is 6 to 8 weeks [16]
Scenario One – Static
YUMA almanac date: Determined upon delivery
Power output: -120 dBm
Description of run: The simulator outputs a signal corresponding to a receiver stationary
at a fixed location Initial position is Lat N34, Lon W118, Height 100m, Duration 3hr [16]
Scenario Two – Racetrack
YUMA almanac date: Determined upon delivery
Power output: -120 dBm
Description of run: The simulator outputs a signal corresponding to a receiver at rest and then moving along an oval track Initial position is Lat N34, Lon W118, Height 100m, Duration 3 hr, velocity 100 m/s, radial acceleration on the turn is 10m/s/s, and each “leg” will be 5 minutes in duration [16]
Signal Dynamics
Maximum Velocity (+ 6000 m/s)
Maximum Acceleration (+ 1000 m/s/s)
Trang 383.2 Welnavigate GS 600 Maximum Jerk (+ 1000 m/s/s/s)
Output Impedance (50 ohms)
Power Level, L1 C/A (- 163 to - 100 dBm)
Trang 393.3 CAST 1000 Satellite Signal Simulator
3.3 CAST 1000 Satellite Signal Simulator
CAST 1000 is a portable satellite signal simulator, in the form of a laptop [17]
The features of CAST 1000 are as follows,
Simulate 10 GPS satellites simultaneously
Simulate both C/A and P codes
Produces both GPS L1 and L2 frequencies
Adjustable RF power output
Built – in, high stability TCCO frequency standard
Individual SV power control
Antenna pattern modeling
Complete scenario generation capability
Differential message output
RAIM events
External trajectory file may be loaded
IP, ICD – GPS – 150 / 153
This laptop functions at the speed of Pentium III 500 MHz with a 10 GB hard drive and 2
GB Jazz drive The dimensions are 17” x 14” x 8” with a weight of 40 lbs It comes with
a SMA female and DB 9 connectors [17]
Specifications
Maximum RF output power (- 105 dBW)
10 Space Vehicles (SVs) [L1 and L2, C/A, P]
Trang 403.3 CAST 1000 Satellite Signal Simulator
Signal Level Control
Range of Power Control – L1 (- 190 to - 135 dBW)
Range of Power Control – L2 (- 190 to - 135 dBW)
Power Control Resolution (0.33 dB)
Maximum Dynamics
Velocity (± 15,000 m/s)
Acceleration (± 3000 m/s/s)
Jerk (± 3000 m/s/s/s)
3.4 Spirent Multi-Channel GPS/SBAS Simulation System STR4500
This is a low cost and compact GPS L1 C/A code and SBAS generation Simulator It consists of 12 independent signal channels that are of high fidelity, accuracy, repeatability and dynamics It comprises of interactive control facilities and multiple vehicle types with comprehensive error effects There are a wide selection of pre-loaded test scenarios; accompany with captured receiver data plus simulation truth data and RTCM-SC104 differential corrections via serial port [18]
The features for this system are as follows,
GPS L1 C/A code and SBAS (WAAS/EGNOS) generation
12 independent signal channels
Low cost portability
High accuracy and repeatability
Multiple vehicle types with comprehensive error models
Wide selection of preloaded test scenarios
Receiver capture and simulation truth data in NMEA-01803 format